When a microprocessor is operating in the process of executing computer software, it is possible for the surreptitious observer to decipher proprietary details regarding processor operation. To protect the computer software, there is often a need to restrict its use. This may be accomplished by end user agreements, wherein the user agrees to only use the software on one or more agreed upon computers. In other cases, the software can be freely copied, so that piracy restrictions are accomplished either by end user agreement or by simple techniques such as the requirement to use a product serial number to enable use of the software. While this may discourage software piracy and copying, it does not completely prevent a determined software pirate from reproducing the program along with one or more enabling serial numbers or keys.
It is possible to specifically identify individual CPUs. In that way, each CPU can be separately identified by a serial number encoded into the CPU. Identification can be accomplished by laser marking of the CPU package with a serial number. Further, it is possible to provide a serial number which can be read out by the end user during execution with appropriate instructions.
It is possible to provide elaborate protective systems for encoding the software, by use of proprietary hardware components for example, or even by requiring the end user to comply with registration requirements in order to enable software operation. In that respect, the encryption scheme for the program ensures that the program is executable in unencrypted form, at least with respect to the instruction sets provided to the CPU. In other words, the instructions provided to the CPU are in a form that is understandable by the CPU prior to CPU execution. Thus, it is easy for an unauthorized user to determine what is necessary to operate the programs successfully.
It is often desired to provide software and updates of software to end users in such a manner that the software is transferred through public channels, such as the Internet. To provide such software in restricted form, it is desired to provide security to the distributor of the software so that the software is not subject to unauthorized use. In particular, if software is shipped via public or private channels, it is desired that the end user of the software can only use the software on the end user's specified computer, and that the software not be willingly or unwillingly shared by the end user. By computer, it is intended that this includes personal computers, smart cards, work stations, dedicated CPUs embedded in hardware appliances, and any other device in which integrated circuit (IC) microprocessors may be used.
In some programs, the cost of the programs to the end user is such that it becomes economical for third parties to determine what is necessary to circumvent restrictions on use by unauthorized persons. Therefore, it is desired to make the unauthorized duplication or use of a program uneconomical. In order to do that, it is desired to provide an encryption scheme which prevents unauthorized persons from “attacking” the encryption of the software through analysis of the input and output of user commands and instruction sets from the software. It is further desired to provide a software encryption technique in which there are no external indicia of a decryption technique which can be used to analyze the encryption of the software. It is further desired that software be encrypted in such a manner that it is unnecessary to decrypt the software in order to accomplish execution of the software.